I believe that's one of several silverplate marks used by Benetfink & Company, Cheapside, London.
The BP mark stands for British Plate, a form of nickel silver.
They used different names throughout their history:
Benetfink & Jones (c. 1845-1851),
Benetfink & Co. (1852-1861),
Benetfink & Fox (1862-1866)
and again Benetfink & Co (1867-c. 1900).
Pat.